1. Field of the Invention
The present invention relates to potatoes and potato plants having a higher starch content per plant body by increasing both the tuber, or underground portion, of the potato plant and the starch content per unit weight of the tuber. The present invention also relates to a method for producing potatoes and a method for producing starch from potatoes.
2. Brief Description of the Related Art
Carbohydrates in plants are mainly present in the form of starch. In Japan, the use of potatoes as a starting material for starch is the highest at about 40%. The use of potatoes as food and in processed foods is 25% and 10%, respectively. The consumption of starch is increasing each year, while the crop yield of potatoes is decreasing. Therefore, there is a need in the art to increase the starch content of potato tubers.
In light of this need, attempts have been made to improve the capacity of carbon dioxide assimilation, as well as the capacity to accumulate substances in the reserve organ so as to synthesize and accumulate an excess amount of saccharides. However, even if the starch content of the tubers can be increased, the amount of tubers is often reduced. The amount of starch calculated per plant body can be increased in only a few cases.
For example, Beaujean et al. introduced C4-phosphoenolpyrvate carboxylase (C4-PEPC) from sorghum and NADP-dependent malate dehydrogenase gene into potatoes in an attempt to increase the carbon dioxide assimilation efficiency of the potatoes. Although an increase in the amount of protein and an increase in the protein activity was observed, the starch content of the tubers did not change (Beaujean et al., Plant Science 160, 1199–1210, 2001). Veramendi et al. introduced the hexokinase 1 gene in an antisense direction into potatoes in an attempt to increase the starch content and succeeded in reducing the hexokinase activity of the potatoes 22% as compared to wild-type strains. As a result, no change in the starch content of the tubers was observed, while the starch content of leaf tissue could be increased 3-fold (Veramendi et al., Plant Physiology 121, 123–134, 1999). When the sucrose transporter gene was introduced in an antisense direction, the yield of tubers was greatly reduced, although the starch content of the leaf tissue was increased 5-fold and thus, starch production could not be increased (Riesmeier J W et al., The EMBO J, 13, 1–7, 1994). Thus, it is difficult to increase starch content of potatoes and the overall yield of tubers thereof at the same time. Therefore, there are no reports to date that the amount of starch per plant body can be increased.